Unsaturated polyester resin compositions, in particular, bulk molding compounds (BMCs) are used to provide a cured product excellent in dimensional accuracy, mechanical characteristics, and flowability, and thus are widely used as an encapsulating material for motors, coils, power generators, and the like to be used in various fields.
Further, for a product such as a motor whose performance reduced by heat generation is considered a problem, the use of an epoxy resin composition comprising an inorganic filler having high thermal conductivity as an encapsulating material from the viewpoint of improving heat dissipation properties has been known (for example, Patent Document 1).
However, the amount of inorganic filler that can be blended in an epoxy resin composition is smaller than what can be blended in an unsaturated polyester resin composition. Therefore, there are problems in epoxy resin compositions in that cured products having sufficient thermal conductivity cannot be obtained, that they require after-cure, and that a cured product thereof has a high mold shrinkage ratio and therefore is easily cracked. As described above, there are a number of problems in epoxy resin compositions regarding moldability, workability, physical properties of a cured product, and the like as compared to unsaturated polyester resin compositions.
On the other hand, unsaturated polyester resin compositions may be easily kneaded and manufactured with high-load manufacturing equipment, and therefore may be blended with a large amount of an inorganic filler having high thermal conductivity as compared to epoxy resin compositions. Further, unsaturated polyester resin compositions can be subjected to closed molding using a molding machine (for example, an injection molding machine or a transfer molding machine) and a mold and also have an advantage of not requiring any post-processing such as after-cure.
Further, there has been a demand for improvement in curing properties of unsaturated polyester resin compositions so as to achieve an increase in productivity of encapsulated products. To meet the demand above, Patent Documents 2 and 3 disclose unsaturated polyester resin compositions comprising a specific curing agent.